The subject invention generally concerns cooling systems for electronic equipment and in particular concerns a variable fan-speed control for electronic test equipment.
Fans are commonly used to provide an adequate supply of cooling air to the interior of an electronic unit, such as a desktop computer, to prevent the unit from overheating. How much air is adequate depends on several factors including the heat generated by the internal components, the ambient temperature at the location where the instrument happens to be operating, and the altitude at which the unit happens to be operating. Clearly, a manufacturer cannot generally control the conditions at the location where the unit will be operated. Therefore, if fixed-speed fans are employed for this purpose, then one must design for worst case conditions. Unfortunately, fixed speed fans selected to handle worst case conditions must move large quantities of air and therefore tend to be noisy. In an office environment, a noisy fan in a computer or other instrument is undesirable.
It has been recognized that those users who operate equipment in worse case conditions are only a small percentage of the total number of users. In most cases, variable speed fans could be substituted for fixed-speed fans, and the variable-speed fans could be operated at a slower, and quieter, speed.
U.S. Pat. No. 5,659,290 (Kundert), issued Apr. 21, 1987, discloses variable-speed fan having a sensor in the exhaust air stream. The fan of Kundert operates at a fixed rate of approximately half-speed until a given temperature is reached, and thereafter variably controls airflow through the fan. While this arrangement does react to some extent to internal temperatures, it assumes that half-speed is the minimum operating fan speed that is needed to effectively cool the instrument. That is, the fan of Kundert will run too fast (and consequently too noisily) if the fan speed actually required is less than half-speed. Moreover, Kundert does not directly monitor ambient air temperature.
Another variable-speed fan system is disclosed in U.S. Pat. No. 5,287,244 (Hileman, et al.), issued Feb. 15, 1994. Hileman, et al. adjust the fan speed in response to an input from an ambient air temperature sensor. Hileman, et al. describe a system that is operating in an open-loop manner. That is, if the ambient air temperature outside of the instrument increases, then the fan speed increases to move more cooling air across the components. While this too is an improvement over fixed-speed fan systems in general noise reduction, apparatus according to this patent exhibits a problem of its own because it does not sense internal case temperatures.
Because the apparatus of Hileman, et al. operates by reacting only to the change of ambient temperature, a problem would arise if, for example, a user places a book next to the instrument, thereby inadvertently partially blocking an air-input port. This condition would reduce airflow, causing internal case temperatures to rise. The fan speed would not increase because the ambient temperature did not change, resulting in overheating of the unit.
On the other hand, a decrease in internal temperature (perhaps, due to reduced power dissipation in a particular component) would require less airflow (with a corresponding decrease in fan noise). The apparatus of Hileman, et al. would fail to react to this condition also, and would operate at a greater than required fan speed and noise level.
In contrast to desktop computer equipment, there is a more stringent requirement to hold temperature constant in test and measurement instruments. It should be noted that without control, the temperature of a component would rise as the ambient air temperature rises in a substantially linear fashion. By use of cooling airflow control, the component temperature can be held substantially constant over a range of ambient temperatures (until the limit of the fan speed is reached). It is desirable that the temperature of certain critical components be held constant, because fluctuations in the temperature of these components may induce errors in the signal measurement process (i.e., adversely affect the calibration of the instrument. Neither of the above noted patents provides the solution to the problem of maintaining constant temperature to the extent required by test and measurement instruments while also reducing fan noise.
There is yet a further requirement imposed by modern manufacturing design in that it is desirable to use the same instrument platform (i.e., case and chassis components) in multiple product lines. This requirement is primarily driven by the need to reduce tooling costs for injection molded plastic parts of the instrument platform. Thus, for example, a logic analyzer may use most of the instrument platform components that are also used in the manufacturing of an oscilloscope, including the cooling system. Unfortunately, the cooling requirements of a logic analyzer tend to be very different from the cooling requirements of an oscilloscope. Moreover, the cooling requirements of each vary as different options are added to the basic instrument. What is needed is a cooling system that can be used for both of the above-noted applications. Such a system would need to provide adequate airflow for cooling for each application without providing excessive airflow in either of them, and would need to operate at reduced noise levels in each application.
A cooling system for a test and measurement instrument including a variable-speed fan, useful with different instruments having different configurations, employs a plurality of sensors disposed at predetermined locations within an enclosure of the instrument, and is software programmable with predetermined parameters of each particular configuration of each particular instrument. At least one of the sensors measures the temperature of a component that dissipates power at a substantially constant rate, and at least one of the sensors measures ambient air temperature, and does not include a heat source for providing heat to be measured.